A month after it last patched Safari, Apple today plugged six security holes, four of them critical, in its Mac and Windows Web browser.

Safari 4.0.3 fixes six flaws in the Windows XP and Vista edition, but only four in the Mac OS X edition. Three of the half-dozen bugs were in WebKit, the open-source browser engine that powers Safari, as well as Google's Chrome.

Four of the vulnerabilities patched today were described by Apple as possibly allowing "arbitrary code execution," company-speak for a critical bug that, if exploited, could let hackers dump malicious software on the machine or hijack it for their own use.

The most unusual flaw is in "Top Sites", a feature Apple introduced in Safari 4.0 that presents users with thumbnails of frequently-visited sites when they launch the browser or open a new tab.

"It is possible for a malicious website to promote arbitrary sites into the Top Sites view through automated actions," warned Apple in its accompany security advisory. "This could be used to facilitate a phishing attack." Apple fixed the flaw by preventing automated site visits from affecting the Top Sites list: http://support.apple.com/kb/HT3733

Other bugs that 4.0.3 addresses include heap and buffer overflows in the Windows version caused by too-long text strings and image metadata handling errors, respectively; a buffer overflow in WebKit's parsing of floating point numbers; a problem with plug-ins that could let hackers steal confidential information; and a URL-spoofing vulnerability that could be exploited by identity thieves.

Apple last patched Safari in early July, about a month after the company officially launched the browser at its annual developers conference in San Francisco.

Safari 4.0.3 for Windows or Mac can be downloaded from Apple's site; current users of the browser can obtain the new version by running Software Update on the Mac, or the bundled Apple Software Update on Windows.

According to Web metrics company Net Applications, Safari had an estimated worldwide market share of 4.1% during July.